# Cosmology | Lecture 1 | Summary and Q&A

## Summary

This video introduces the concept of the expanding universe and the geometry of space in modern cosmology. It explains the basic Big Bang Theory and the three possible versions of it. It also discusses the Hubble law and the relation between velocity and distance. The video touches on concepts such as the scale factor and the Hubble constant, as well as the isotropy and homogeneity of the universe. The video concludes by mentioning other topics in modern cosmology, such as inflation, dark energy, dark matter, and the idea of a multiverse.

## Questions & Answers

### Q: What are the basics of modern cosmology?

The basics of modern cosmology consist of the Big Bang Theory, the Hubble law, and the general principles of cosmology known as Big Bang cosmology.

### Q: What does the Hubble law state?

The Hubble law states that the velocity at which galaxies are moving away from us is proportional to their distance from us. The proportionality constant is known as the Hubble constant.

### Q: How is the velocity between two galaxies calculated?

The velocity between two galaxies is calculated by multiplying the distance between them by the Hubble constant. The Hubble constant is a measure of how fast the universe is expanding.

### Q: Does the Hubble constant depend on which galaxies we are talking about?

No, the Hubble constant does not depend on the specific galaxies being considered. It is the same for every galaxy, as it represents the rate at which the universe is expanding.

### Q: Can the velocity between galaxies change with time?

Yes, the velocity between galaxies can change with time depending on how the scale factor of the universe changes. The scale factor is a measure of the distance between neighboring galaxies.

### Q: What is the metric of space in a one-dimensional universe?

In a one-dimensional universe, the metric is given by "a squared," where "a" represents the scale factor. This metric determines the distance between neighboring points in space.

### Q: How does the spatial distance factor into the proper time of a particle?

The square of the spatial distance between two neighboring points in space is subtracted from the square of the time interval to obtain the proper time along a particle's trajectory.

### Q: What does it mean for the universe to be homogeneous and isotropic?

Homogeneity means that the universe is the same everywhere, with equal spacing between galaxies. Isotropy means that the universe looks the same in every direction when viewed on a large scale.

### Q: How did Hubble estimate the distance to galaxies?

Hubble estimated the distance to galaxies based on their size and brightness. Bright and big galaxies were assumed to be closer, while small and dim galaxies were assumed to be farther away.

### Q: Has the estimated age of the universe been corrected?

Yes, initially Hubble's estimate of the age of the universe was off by a factor of 10 due to an incorrect measurement of distances to galaxies. The age of the universe has been corrected since then.

### Q: What other topics are addressed in modern cosmology?

Modern cosmology also explores topics such as inflation, dark energy, dark matter, the cosmological constant, the accelerated expansion of the universe, and the idea of a multiverse.

## Takeaways

In this video, we learned about the basics of modern cosmology, including the Big Bang Theory, the Hubble law, and the geometry of the expanding universe. The Hubble law states that the velocity of galaxies is proportional to their distance from us, and the Hubble constant represents this proportionality. The metric of space in a one-dimensional universe is given by the scale factor. We also discussed the concepts of homogeneity and isotropy, where the universe is the same everywhere and looks the same in every direction when viewed on a large scale. Other topics in modern cosmology include inflation, dark energy, dark matter, and the possibility of a multiverse.